1. Technical Field
The invention relates to cable television (CATV) interdiction systems and, more particularly, to a method and apparatus for controlling the jamming parameters in such systems.
2. Description of the Prior Art
In the past, a scrambler has been provided to encode premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/decoder at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the decoder portion of the converter/decoder to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/ decoder at the premises of the television receiver was particularly expensive. Furthermore, providing a converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols, which in some instances involve multiple layers of encryption by both in-band and out-of-band data transmissions thereby further increasing the costs of the converter/decoder. In addition, scrambling systems may leave artifacts in the final signal.
Consequently, the cable industry has reviewed other technology developed in the early stages of cable television, such as the application of negative and positive traps, and more recent techniques, such as interdiction, to improve CATV systems.
A relatively recent technique for premium channel control is the interdiction system, so called because of the introduction of an interfering signal into a premium channel at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve one or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For the sake of efficiency, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The jamming signal frequency is moved as a function of time from channel to channel. The oscillator is frequency agile and hops from jamming one premium channel frequency to the next. Cable television channels and, of course, premium service channels may extend over a wide range of frequencies, for example, from 54 Mhz. up to and including 1000 Mhz. Thus, if only one oscillator were provided, it would have to be frequency agile over a wide range.
One such system, illustrated in U.S. Pat. No. 4,450,481 by Dickenson, has a single frequency agile oscillator which provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal of approximately the same frequency has been superimposed.
A significantly more advantageous interdiction system is disclosed in U.S. Pat. No, 4,912,760 by West, Jr., et al. which controls a plurality of frequency agile oscillators for each subscriber. Each voltage controlled oscillator is allocated a continuous band of frequencies consistent with the elimination of jamming signal harmonics which could disturb authorized programming at a higher channel frequency. The interdiction apparatus includes generating and storing frequency control words for operating the voltage controlled oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data. In U.S. patent application Ser. No. 07/476,041, filed Feb. 6, 1990, by West, Jr., et al., the method of programming the time slots of the West I system to vary the dwell time and jamming factor of the jamming signals for different premium channels was disclosed.
West I and II disclose a method of calibrating the frequency control words stored in the frequency control memory. The calibration is necessary because the same digital number may produce significantly different output frequencies from the different subscriber modules or even from different voltage controlled oscillators within the same subscriber module. This happens because of tolerances in the circuitry, aging of components, environmental changes, etc. To counteract this problem a calibration technique is used which includes loading a frequency control word in the frequency control memory which is representative of the frequency desired and measuring the actual output frequency of the selected voltage controlled oscillator. Another frequency control word is then loaded into the memory which narrows the difference between the actual output frequency of the VCO and the desired output frequency. The process is repeated until a precise output frequency of the desired tolerance is obtained. While this process can be accomplished relatively quickly, several calibration cycles are necessary for each frequency control word which is loaded into memory.
Manifestly, the sum of all the frequency control words which are stored in the frequency control memory and the sequence in which they are read out from memory comprise the jamming pattern for a particular subscriber. It was indicated in West I and II that it was desirable to be able to vary the jamming pattern for a subscriber on a programmable basis which is additionally addressable by a headend of a cable system. The frequency control word memory of West I and R provides this function but to change the jamming pattern any new frequency control words must have to be recalibrated.
In addition, the headend has to generate an addressable transaction, that is a message directed to an individual subscriber module, which for a several hundred thousand subscriber system may require complex and relatively lengthy communications. While this is not a considerable concern when a subscriber first becomes authorized for service, changes between tiers of service, or is even deauthorized from service. It does present a more difficult challenge when the system operation jamming patterns need to be changed for many subscribers for only a relatively short period of time and in an individualized manner.
Recent service advances have required this nontypical system operation. The best examples of this type of activity are a pay per view events, where initially the jamming patterns of all subscribers deny reception of a particular channel. When subscribers call in or communicate with the system operator to buy an event, the frequency control word which jams that premium channel must be removed from one or several time slots for each subscriber which takes the event and those jamming slots reallocated to other channels during the event. Thereafter, the frequency control word representing the premium channel on which the event was shown must be reloaded into the one or more time slots of the frequency control memory and those frequency control words recalibrated.
Another type of service area where such problems arise are new intermittent services, such as being provided with service only on weekends or for other short contiguous blocks of time such as only after 6 P.M. to midnight, etc. To allow subscribers the most flexibility in their choice of such new services different tier choices should also be made available. This requires not only a single channel change in jamming patterns, such as in PPV event type buys, but also tier changes in jamming patterns and different times of change for individual subscribers. As in the PPV event type service, large numbers of subscribers may need to be authorized or deauthorized at the same time. It would be much more advantageous to be able to do this with global transactions rather than an addressable local transaction.
Therefore, it would be to advantage to be able to load a basic jamming pattern in a frequency control memory of an interdiction system and then be able to vary that jamming pattern flexibly for relatively short periods of time on an individual basis. After such intermittent use it would be advantageous to permit the system to revert to the basic jamming pattern without the need of an addressable transaction and without the necessity of recalibration of that basic pattern.